Believed to have originated in the Mediterranean region, the
onion thrips is cosmopolitan in distribution throughout most of
the world. This thrips was first found in Hawaii in 1915 and is
now present on all islands.

Larvae and adults are found mainly in the narrow space between
the tubular leaves of onions, in flowers and on the underside of
foliage of certain other plants. Thrips feed by piercing
individual cells and sucking the contents. These cells lose their
normal color, and when many adjacent cells are damaged, the
tissue appears as whitish spots or silvery spots or streaks. In
advanced injury the leaves take on a blasted appearance. As is
common with many thrips species, onion thrips deposit small dark
specks of excrement on the surface of tissue where they feed.
Substantial damage can be done to young plants especially to
varieties grown in seed beds.

Besides direct damage caused by feeding of larvae and adults,
this pest is also important as a vector of tomato spotted wilt
virus and has been involved in transmitting the disease in
pineapple, tomatoes, and certain other crops.

Stages in the developmental cycle are the egg, first larval
stage, second larval stage, prepupa, pupa, and adult. Because of
their small size, this pest species like other thrips cannot
readily be identified to species even with a hand lens. Adult
specimens are usually needed to make species identifications
under high microscope magnification.

The entire life cycle (egg to adult) requires about 19 days.
Large populations are able to develop quickly under Hawaiian
weather conditions where there are many overlapping generations
throughout the year.

Reproduction of this species in Hawaii is mostly through a
process called parthenogenesis in which females are able to
reproduce without mating. As a result, populations consist of
females at a ratio of 1 male per 1000 females (Sakimura, 1932).

EGGS

Females have a saw-like structure that helps to make an
incision in plant tissue for egg laying. Eggs are placed singly
just under the epidermis of succulent leaf, flower, stem or bulb
tissue. Eggs are elliptical, approximately 1/125 inch in length.
They are whitish at deposition and change to an orange tint as
development continues. Hatching occurs in 4-5 days in Hawaii.

LARVAE

Larvae are whitish to yellowish. There are two larval stages
and besides the adults they are the only damaging stages. Larval
development is completed in about 9 days.

PUPAE

There are two non- feeding stages called the prepupa and pupa.
They do not feed and occur primarily in the soil. Combined
prepupal and pupal development is completed in 4-7 days.

ADULTS

Adults
are 1/25 inch long. Their body color ranges from pale yellow to
dark brown; wings are unbanded and dirty gray. In Hawaii, this
species has a darker form during the rainy season. Males are
wingless and exceedingly rare. Females live for about two to
three weeks and each can lay about 80 eggs.

Several natural enemies have been introduced to Hawaii in an
attempt to help control this pest. However, only the parasite,
Ceranisus menes, has become established. Unfortunately the impact
of this parasite is not considered great.

Cultural Control

Cultural control practices can help reduce onion thrips
infestations. Destruction of piles of cull onions between crops
reduces the abundance of this pest along with other sanitation
techniques such as removing weeds in the field and outlying
areas. These practices help eliminate alternative hosts on the
onion thrips between crops. Crop rotations to prevent the
successive plantings of several onion crops and interplanting
with non-host crops can also be effective in deterring large
populations.

CHEMICAL CONTROL

While the onion thrips can be readily killed by many
insecticides, they are often difficult to control because of
their small size and cryptic habits. Insecticidal control of this
pest depends on the choice of an effective chemical and adequate
spray coverage on parts of the plant where the thrips inhabit.
For example, control of this pest on onions is exceedingly
difficult because of the shape of onion leaves. Many larvae and
thrips are found in the leaf axils which often do not receive
insecticide deposits.

Thrips are shallow feeders that feed primarily on surface
tissue. Contact-residual insecticides are more effective.
Systemic insecticides which transport through the plants vascular
tissues are not as likely to be effective if they are not applied
in a manner like other contact insecticides.

Another important aspect of insecticidal control is spray
intervals. Since eggs, prepupae, and pupae are not easily killed
by insecticidal sprays, the time required for egg incubation and
larval development must be considered. In order to control high
population densities of this pest during warm, dry periods,
repeated applications at 7-10 day intervals are required.

Zimmerman, E. C. 1948. Thrips (Thrips) tabaci Lindeman. pp.
422-425. In Insects of Hawaii. A Manual of the Insects of the
Hawaiian Islands, including Enumeration of the Species and notes
on the Origin, Distribution, Hosts, Parasites, etc. volume. 2:
Apterygota to Thysanoptera. The University Press of Hawaii,
Honolulu. 475 pages.